It may be added, that, whereas the logical form of this argument, is, as I have already observed, indirect, viz. that “the conclusion cannot be otherwise,” and Butler says that an event is proved, if its antecedents “could not in reason be supposed to have happened _unless_ it were true,” and law-books tell us that the principle of circumstantial evidence is the _reductio ad absurdum_, so Newton is forced to the same mode of proof for the establishment of his lemma, about prime and ultimate ratios. “If you deny that they become ultimately equal,” he says, “let them be ultimately unequal;” and the consequence follows, “which is against the supposition.” Such being the character of the mental process in concrete reasoning, I should wish to adduce some good instances of it in illustration, instances in which the person reasoning confesses that he is reasoning on this very process, as I have been stating it; but these are difficult to find, from the very circumstance that the process from first to last is carried on as much without words as with them. However, I will set down three such. 1. First, an instance in physics. Wood, treating of the laws of motion, thus describes the line of reasoning by which the mind is certified of them. “They are not indeed self-evident, nor do they admit of accurate proof by experiment, on account of the effects of friction and the air’s resistance, which cannot entirely be removed. They are, however, constantly and invariably suggested to our senses, and they agree with experiment, as far as experiment can go; and the more accurately the experiments are made, and the greater care we take to remove all those impediments which tend to render the conclusions erroneous, the more nearly do the experiments coincide with these laws. “Their truth is also established upon a different ground: from these general principles innumerable particular conclusions have been deducted; sometimes the deductions are simple and immediate, sometimes they are made by tedious and intricate operations; yet they are all, without exception, consistent with each other and with experiment. It follows thereby, that the principles upon which the calculations are founded are true.(23)”